Phenotypic drug susceptibility assay for influenza virus neuraminidase inhibitors

A flow cytometric (fluorescence-activated cell sorter [FACS]) assay was developed for analysis of the drug susceptibilities of wild-type and drug-resistant influenza A and B virus laboratory strains and clinical isolates for the neuraminidase (NA) inhibitors oseltamivir carboxylate, zanamivir, and peramivir. The drug susceptibilities of wild-type influenza viruses and those with mutations in the hemagglutinin (HA) and/or NA genes rendering them resistant to one or more of the NA inhibitors were easily determined with the FACS assay. The drug concentrations that reduced the number of virus-infected cells or the number of PFU by 50% as determined by the FACS assay were similar to those obtained with the more time-consuming and labor-intensive virus yield reduction assay. The NA inhibition (NAI) assay confirmed the resistance patterns demonstrated by the FACS and virus yield assays for drug-resistant influenza viruses with mutations in the NA gene. However, only the FACS and virus yield assays detected NA inhibitor-resistant influenza viruses with mutations in the HA gene but not in the NA gene. The FACS assay is more rapid and less labor-intensive than the virus yield assay and just as quantitative. The FACS assay determines the drug susceptibilities of influenza viruses with mutations in either the HA or NA genes, making the assay more broadly useful than the NAI assay for measuring the in vitro susceptibilities of influenza viruses for NA inhibitors. However, since only viruses with mutations in the NA gene that lead to resistance to the NA inhibitors correlate with clinical resistance, this in vitro assay should not be used in the clinical setting to determine resistance to NA inhibitors. The assay may be useful for determining the in vivo susceptibilities of other compounds effective against influenza A and B viruses.

[Neuraminidase inhibitor, anti-influenzal agent--mechanism of action, and how to use clinically]

Neuraminidase inhibitor has changed the treatment of influenza dramatically. The drug is effective for both influenza A and B whereas amantadine, another anti-influenzal agent, inhibits influenza A virus infection but not B. Neuraminidase inhibitor blocks the process of release of influenza virus from infected cells and inhibit the virus transmission to the neighboring cells. Neuraminidase inhibitor improves the symptoms of influenza and shortens the duration of illness. It is emphasized that this drug should be given within forty-eight hours after the onset, and it must be not noted that the possibility of the drug resistant virus although it is observed infrequently. Prophylactic use of the drug is also discussed.

Effect of a single mutation in neuraminidase on the properties of Influenza B virus isolates

Two strains of Influenza B virus were isolated in Vero cells. Subclones with improved efficiency of plaque formation were selected. The activity of the neuraminidase (NA) of the two subclones compared to their respective isolates dropped 20- and 100-fold, respectively. Both subclones had a common mutation in segment 6 leading to a change from Asp to Asn at position 457 in the NA. This mutation destroyed a salt bridge of the contact surface between the monomers, thereby causing the loss in enzymatic activity. The decreased NA activity caused improved plaque formation but had no significant impact on the replication in liquid culture.

Pyrrolidinobenzoic acid inhibitors of influenza virus neuraminidase: modifications of essential pyrrolidinone ring substituents

We recently reported the first benzoic acid, 1-[4-carboxy-2-(3-pentylamino)phenyl]-5,5-bis(hydroxymethyl)pyrrolidin-2-one (8), that is a potent inhibitor of avian influenza A neuraminidase (N9) and, unlike other reported potent neuraminidase inhibitors, does not contain a basic aliphatic amine or guanidine nor a simple N-acetyl grouping. However, 8 was a poor inhibitor of influenza B neuraminidase. In the present study we further evaluated 8 as an inhibitor of human influenza A NA isolates, and it was effective against N2NA but found to be 160-fold less active against N1NA. We also synthesized analogues of 8 involving moderate modifications of essential substituents on the pyrrolidinone ring. Specifically, the aminomethyl (9), hydroxyethyl (10), and aminoethyl (11) analogues were prepared. Only the most conservative change (compound 9) resulted in continued effective inhibition of influenza A, in addition to a noteworthy increase in the activity of 9 for N1NA. The effectiveness of 9 against influenza B neuraminidase was furthermore improved 10-fold relative to 8, but this activity remained 50-fold poorer than for type A NA.

A point mutation in influenza B neuraminidase confers resistance to peramivir and loss of slow binding

The influenza neuraminidase (NA) inhibitors peramivir, oseltamivir, and zanamivir are potent inhibitors of NAs from both influenza A and B strains. In general, these inhibitors are slow, tight binders of NA, exhibiting time-dependent inhibition. A mutant of influenza virus B/Yamagata/16/88 which was resistant to peramivir was generated by passage of the virus in tissue culture, in the presence of increasing concentrations (0.1-120 microM over 15 passages) of the compound. Whereas the wild type (WT) virus was inhibited by peramivir with an EC(50) value of 0.10 microM, virus isolated at passages 3 and 15 displayed EC(50) values of 10 and >50 microM, respectively. Passage 3 virus contained 3 hemagglutinin (HA) mutations, but no NA mutation. Passage 15 (P15R) virus contained an additional 3 HA mutations, plus the NA mutation His273Tyr. The mechanism of inhibition of WT and P15R NA by peramivir was examined in enzyme assays. The WT and P15R NAs displayed IC(50) values of 8.4+/-0.4 and 127+/-16 nM, respectively, for peramivir. Peramivir inhibited the WT enzyme in a time-dependent fashion, with a K(i) value of 0.066+/-0.002nM. In contrast, the P15R enzyme did not display the property of slow binding and was inhibited competitively with a K(i) value of 4.69+/-0.44nM. Molecular modeling suggested that His273 was relatively distant from peramivir (>5A) in the NA active site, but that Tyr273 introduced a repulsive interaction between the enzyme and inhibitor, which may have been responsible for peramivir resistance.

ZstatFlu-II test: a chemiluminescent neuraminidase assay for influenza viral diagnostics

The ZstatFlu-II test is a highly sensitive, specific, rapid, point-of-care chemiluminescent diagnostic test for influenza infection. Influenza viral neuraminidase-specific substrate, spiroadamantyl-1,2-dioxetane-4,7-dimethoxy-N-acetyl-neuraminic acid, is at the core of the ZstatFlu-II Test. The enzymatic reaction was carried out at 25 degrees C and neutral pH, representing the optimum assay conditions for influenza types A and B viral neuraminidases. The results were outputted on a Polaroid trade mark High Speed Detector Film. Positive results appeared as a '+'-shaped white film image; negative results produced no image. The 'glow' kinetics, facilitated by a unique combination of light enhancers, also 'tuned' the wavelength of emission to match the spectral properties of the film. The substrate hydrolysed non-enzymatically at acid pH or at temperatures above 25 degrees C. In order to minimize false positives, the ZstatFlu-II Test was formatted with 0.3-0.4 K(m) substrate and freezing the test kit until use. The pH optimization of the ZstatFlu-II test is discussed with reference to model compounds of sialyl-glycosides. A nucleophilic attack or an electrostatic stabilization of a developing carbonium ion under the influence of the adjacent carboxyl group was probably responsible for non-enzymatic hydrolysis of the substrate. Intramolecular general acid catalysis is proposed as a mechanism for the lability of the O-glycosidic linkage of the substrate.

Use of a pharmacophore model to discover a new class of influenza endonuclease inhibitors

Data from both our own and literature studies of the biochemistry and inhibition of influenza virus endonuclease was combined with data on the mechanism of action and the likely active site mechanism to propose a pharmacophore. The pharmacophore was used to design a novel structural class of inhibitors, some of which were found to have activities similar to that of known influenza endonuclease inhibitors and were also antiviral in cell culture.

Sensitivity of molecular docking to induced fit effects in influenza virus neuraminidase

Many proteins undergo small side chain or even backbone movements on binding of different ligands into the same protein structure. This is known as induced fit and is potentially problematic for virtual screening of databases against protein targets. In this report we investigate the limits of the rigid protein approximation used by the docking program, GOLD, through cross-docking using protein structures of influenza neuraminidase. Neuraminidase is known to exhibit small but significant induced fit effects on ligand binding. Some neuraminidase crystal structures caused concern due to the bound ligand conformation and GOLD performed poorly on these complexes. A 'clean' set, which contained unique, unambiguous complexes, was defined. For this set, the lowest energy structure was correctly docked (i.e. RMSD < 1.5 A away from the crystal reference structure) in 84% of proteins, and the most promiscuous protein (1mwe) was able to dock all 15 ligands accurately including those that normally required an induced fit movement. This is considerably better than the 70% success rate seen with GOLD against general validation sets. Inclusion of specific water molecules involved in water-mediated hydrogen bonds did not significantly improve the docking performance for ligands that formed water-mediated contacts but it did prevent docking of ligands that displaced these waters. Our data supports the use of a single protein structure for virtual screening with GOLD in some applications involving induced fit effects, although care must be taken to identify the protein structure that performs best against a wide variety of ligands. The performance of GOLD was significantly better than the GOLD implementation of ChemScore and the reasons for this are discussed. Overall, GOLD has shown itself to be an extremely good, robust docking program for this system.

A reverse genetics approach for recovery of recombinant influenza B viruses entirely from cDNA

The recovery of recombinant influenza A virus entirely from cDNA was recently described (9, 19). We adapted the technique for engineering influenza B virus and generated a mutant bearing an amino acid change E116G in the viral neuraminidase which was resistant in vitro to the neuraminidase inhibitor zanamivir. The method also facilitates rapid isolation of single-gene reassortants suitable as vaccine seeds and will aid further investigations of unique features of influenza B virus.

[A novel test for diagnosis of influenza]

OBJECTIVE

To set up a novel, simple, sensitive, specific, repeatable and rapid assay for diagnosis of influenza.

METHODS

Monolayers of MDCK cells were inoculated with the specimens for amplifying viral yield, the feature of receptors on cell surface was changed by treatment of neuraminidases of influenza A and B viruses. Afterward, based on the lectin binds to receptors on cell surface with strict specificity,the phenomenon of red blood cell aggregation was observed under the conventional microscope. Finally, the tested results could be determined by the extent of red blood cell aggregation.

RESULTS

There was a complete (%) consistency rate (100%) for viral isolation between new and routine tests. In general, the results were detected with new assay within 20 h. The sensitivity of new assay was over 100-10,000 times higher than that of routine method. Meanwhile, the novel test could not only be used for rapid diagnosis in the clinic, but also be used for influenza surveillance. The best concentration of red blood cells was 1 in the detection assay. The testing result was not effected by red blood cells taken from either different red blood cell type of human or different individual of guinea pigs.

CONCLUSIONS

The novel method has several advantages: simple, high sensitivity and specificity, accurate and suitable for multiple purposes.

Synthesis and anti-influenza virus activity of 4-guanidino-7-substituted Neu5Ac2en derivatives

Substitution of 7-OH by small hydrophobic groups on zanamivir resulted in the retaining of low nanomolar inhibitory activities against not only influenza A virus sialidase but also influenza A virus in cell culture. These compounds were prepared by treatment of the corresponding 7-substituted sialic acids derived from 4-modified N-acetyl-D-mannosamine (ManNAc) using enzyme-catalyzed aldol condensation.

Susceptibility of recent Canadian influenza A and B virus isolates to different neuraminidase inhibitors

Forty-two influenza A and 23 influenza B isolates collected from untreated subjects during the 1999-2000 influenza season in Canada were tested for their susceptibility to three neuraminidase inhibitors (zanamivir, oseltamivir carboxylate and RWJ-270201 or BCX-1812) using a chemiluminescent neuraminidase assay. Influenza B isolates were less susceptible than A viruses to all tested drugs. RWJ-270201 was the most potent drug against both influenza A(H3N2) (mean IC(50): 0.60 nM) and B (mean IC(50): 0.87 nM) viruses. Oseltamivir carboxylate was more active than zanamivir for influenza A(H3N2) isolates (mean IC(50): 0.73 vs. 2.09 nM) whereas it was less potent against B viruses (mean IC(50): 11.53 vs. 4.15 nM).

In vitro characterization of A-315675, a highly potent inhibitor of A and B strain influenza virus neuraminidases and influenza virus replication

A-315675 is a novel, pyrrolidine-based compound that was evaluated in this study for its ability to inhibit A and B strain influenza virus neuraminidases in enzyme assays and influenza virus replication in cell culture. A-315675 effectively inhibited influenza A N1, N2, and N9 and B strain neuraminidases with inhibitor constant (K(i)) values between 0.024 and 0.31 nM. These values were comparable to or lower than the K(i) values measured for oseltamivir carboxylate (GS4071), zanamivir, and BCX-1812, except for the N1 enzymes that were found to be the most sensitive to BCX-1812. The time-dependent inhibition of neuraminidase catalytic activity observed with A-315675 is likely due to its very low rate of dissociation from the active site of neuraminidase. The half times for dissociation of A-315675 from B/Memphis/3/89 and A/Tokyo/3/67 (H3N2) influenza virus neuraminidases of 10 to 12 h are significantly slower than the half times measured for oseltamivir carboxylate (33 to 60 min). A-315675 inhibited the replication of several laboratory strains of influenza virus in cell culture with potencies that were comparable or superior to those for oseltamivir carboxylate and BCX-1812, except for the A/H1N1 viruses that were found to be two- to fourfold more susceptible to BCX-1812. A-315675 and oseltamivir carboxylate exhibited comparable potencies against a panel of A/H1N1 and A/H3N2 influenza virus clinical isolates, but A-315675 was found to be significantly more potent than oseltamivir carboxylate against the B strain isolates. The favorable in vitro results relative to other clinically effective agents provide strong support for the further investigation of A-315675 as a potential therapy for influenza virus infections.

The effect of zanamivir treatment on the early immune response to influenza vaccination

Zanamivir is licensed for influenza treatment, but may also play a role in prophylaxis either alone or in combination with vaccine in epidemic periods. We conducted a double blind placebo controlled trial to investigate the effect of zanamivir treatment on the humoral immune response to influenza vaccine. Forty young healthy volunteers were vaccinated with licensed trivalent influenza vaccine and received 20 mg zanamivir (24 subjects) or placebo (16 subjects) daily for a period of 14 days. No significant differences were observed in the magnitude or the time course of the antibody response to the influenza H3N2 and B strains between the two groups, in contrast the placebo group responded with higher antibody titres to the H1N1. Our results suggest that during an influenza epidemic, volunteers would only need to continue zanamivir treatment for the initial 12 days after vaccination whilst the vaccine induced protective antibody response developed.

Aromatic sialic acid analogues as potential inhibitors of influenza virus neuraminidase

The influenza virus neuraminidase (NA) is an enzyme essential for viral infection and offers a potential target for antiviral drug development. We aimed our research at the synthesis of non-carbohydrate molecules able to inhibit NA as transition-state analogues. Aromatic sialic acid analogues (compound 5 and compound 10) were synthesised in good yields starting from commercially available benzoic acids using a suitable synthetic strategy.

Sialidase inhibitors related to zanamivir. Further SAR studies of 4-amino-4H-pyran-2-carboxylic acid-6-propylamides

SAR investigations of the 4- and 5-positions of a series of 4-amino-4H-pyran-2-carboxylic acid 6-carboxamides are reported. Potent inhibitors of influenza A sialidase with marked selectivity over the influenza B enzyme were obtained when the basic 4-amino substituent was replaced by hydroxyl or even deleted. Modifications at the 5-position exhibited a tight steric requirement, with trifluoroacetamide being optimal.

Zanamivir for the treatment of influenza A and B infection in high-risk patients: a pooled analysis of randomized controlled trials

BACKGROUND

Influenza can cause significant morbidity and mortality, particularly in patients considered to be at high risk (such as the elderly and those with chronic disease) of developing influenza-related complications. Data on the efficacy of zanamivir in high-risk patients are lacking because individual studies recruited a limited number of these patients.

METHODS

A retrospective pooled analysis of data from high-risk patients in studies completed before or during the 1998-1999 winter season was performed to investigate the efficacy and safety of inhaled zanamivir (10 mg twice daily for 5 days) for the treatment of confirmed influenza. All studies were randomized, double-blind, and placebo-controlled with 21- to 28-day follow-up. A total of 2751 patients was recruited. Of these, 321 (12%) were considered high risk and 154 were randomized to zanamivir. The median time to alleviation of influenza symptoms and time to return to normal activities were the main outcome measures.

RESULTS

Zanamivir-treated high-risk patients had a treatment benefit of 2.5 days compared with those given placebo (P = .015). Patients treated with zanamivir returned to normal activities 3.0 days earlier (P = .022) and had an 11% reduction (P = .039) in the median total symptom score over 1 to 5 days relative to those taking placebo. In addition, zanamivir reduced the incidence of complications requiring antibiotic use by 43% relative to placebo users (P = .045). Adverse events reported were of a similar nature and frequency between the two groups.

CONCLUSION

This pooled analysis shows that zanamivir is an effective and well-tolerated treatment for influenza in patients considered at high-risk of developing influenza-related complications.

[The rapid detection kit based on neuraminidase activity of influenza virus]

The ZstatFlu test(ZymeTx, USA) is a rapid detection kit for influenza types A and B virus. This test is based upon the reaction between viral neuraminidase from influenza viruses and chromogenic substrate. The positive specimen of influenza type A or B virus cleave the substrate and produce a blue colored product. The ZstatFlu was evaluated by a prototype viruses, isolated viruses and clinical specimens. At result, this kit was reactive for all human influenza type A and B virus. No cross reactivity was detected with other respiratory viruses, including parainfluenza type 1, 2, 3 and mumps viruses with neuraminidase activity. Throat swabs were used for the test. By comparison with cell culture and RT-PCR. The sensitivity and the specificity was 77.0% and 90.2% respectively. The ZstatFlu should be useful for the rapid diagnosis of influenza virus infection.

Carbocyclic influenza neuraminidase inhibitors possessing a C3-cyclic amine side chain: synthesis and inhibitory activity

As part of our continuing work in the area of influenza neuraminidase inhibitors, a series of C3-aza inhibitors possessing a cyclic amine side chain was synthesized and evaluated for influenza neuraminidase inhibitory activity. Analogues possessing a six-, seven- and eight-membered ring, 4c-e, respectively, at the C3 position exhibited excellent influenza B neuraminidase inhibition.

Comparison of a new neuraminidase detection assay with an enzyme immunoassay, immunofluorescence, and culture for rapid detection of influenza A and B viruses in nasal wash specimens

The performance of a new, rapid, easy-to-perform assay based on neuraminidase enzyme activity for detection of influenza virus types A and B was compared to detection by culture, indirect immunofluorescence, and enzyme immunoassay in 479 nasal wash specimens from children with respiratory infections. Compared to isolation of influenza virus by culture, the neuraminidase assay had a sensitivity of 70.1%, specificity of 92.4%, positive predictive value of 76.3%, and negative predictive value of 89.9%. There was a higher sensitivity for the detection of influenza A virus (76.4%) than for influenza B virus (40.9%). Indirect immunofluorescence showed a sensitivity of 59.8% and specificity of 97% compared to culture isolation for detection of influenza A and B viruses. Enzyme immunoassay showed a sensitivity of 89.7% and specificity of 98.1% for the detection of influenza A alone. The quality of the nasal wash specimen had a significant effect on the detection of influenza virus by all of the assays. A strong response of the neuraminidase assay was more likely to represent a culture-confirmed influenza infection. This new rapid neuraminidase assay was useful for the detection of influenza A and B viruses in nasal wash specimens.

Evaluation of a neuraminidase detection assay for the rapid detection of influenza A and B virus in children

A prototype version of a new diagnostic assay for influenza A and B (Zstat Flutrade mark) based on detection of viral neuraminidase was evaluated and compared to culture in 196 clinical samples. Children with respiratory illnesses were prospectively evaluated at a pediatrician's office and at a large children's hospital using the neuraminidase assay and viral culture performed on respiratory secretions. Influenza virus was isolated from 51 samples and 83 were positive by the neuraminidase assay. When compared to culture the sensitivity of the assay was 96%, specificity was 77%, positive predictive value was 59%, and negative predictive value was 98%. Testing in the laboratory of pure cultures of bacteria and non-influenza viruses frequently found in the respiratory tract showed 0% cross-reactivity with the neuraminidase assay and 100% specificity for influenza virus in vitro. This new assay provided useful information for the preliminary diagnosis of influenza A and B infections and appears to be suitable for both point-of-care use in the physician's office and rapid diagnosis in a virology laboratory. The high sensitivity makes it particularly useful as a screening test for exclusion of influenza A and B infections. To confirm the diagnosis and exclude a false-positive result, as well as to determine the influenza virus type, a viral culture may be considered.

Monitoring of viral susceptibility: new challenges with the development of influenza NA inhibitors

With the clinical development of anti-viral agents, monitoring for the continued susceptibility of wild-type strains has become important in disease management. Various methods have been used to monitor viral susceptibility; the advantages and disadvantages of which depend on the virus, the target and the scale of the research being undertaken. The plaque-reduction assay is valuable for measuring susceptibility of most viruses but is not ideal for large-scale monitoring. Yield-reduction, measuring specific virus antigens, and dye-uptake assays, measuring virus cytopathic effects, are more suitable for high-throughput requirements, but the IC(50) value (the concentration that inhibits 50% of virus) varies with the viral inoculum. Surveillance of influenza susceptibility to rimantadine/amantadine in the clinic has predominantly used EIA-based assays, since plaquing of influenza clinical isolates is variable. With development of the influenza NA inhibitors it became apparent that current cell-based assays were unsuitable for monitoring susceptibility to this new class of drugs. Variability may result from virus spread directly from cell to cell in culture by-passing the NA function. Furthermore, mutations selected in the HA, while not apparently contributing to phenotypic resistance in vivo, may result in cell-culture based resistance, and may mask NA resistance in cell culture by modifying receptor-binding specificity. One important distinction between NA inhibitors and other antiviral enzyme inhibitors is that both target enzyme and inhibitor work extracellularly. NA assays are therefore most representative of the in vivo situation for monitoring susceptibility, supported by HA sequencing. As the clinical use of NA inhibitors escalates, a major change will be required in approaches used to monitor susceptibility of influenza isolates in virology laboratories world-wide.

[Evaluation of the rapid detection test for influenza A and B viruses using neuraminidase activity]

The ZstatFlu test (ZymeTx, USA) is a rapid detection kit for influenza A and B viruses. This test is based upon the reaction between viral neuraminidase from influenza viruses and a chromogenic substrate. The clinical performance of the ZstatFlu test was determined by comparison with viral isolation in cell culture. A total of 176 respiratory specimens from 172 pediatric patients with influenza like illnesses during the 1998/99 season were tested. Influenza viruses were recovered from 97 specimens (type A: 6, type B: 91) in cell culture. ZstatFlu demonstrated 67.4% sensitivity (29/43) and 62.7% specificity (37/59) for throat swabs. Of the 22 ZstatFlu-positive, culture-negative throat swabs tested by RT-PCR, 18 were positive by RT-PCR. ZstatFlu showed 48.1% sensitivity (26/54) and 90.0% specificity (18/20) for nasopharyngeal aspirates. Of the two ZstatFlu-positive, culture-negative nasopharyngeal aspirates tested by HI titer of paired sera, one showed a 4-fold increase of HI titer. Nasopharyngeal aspirates therefore showed lower sensitivity than throat swabs at this test, different from EIA test kits such as Directigen FluA or FLU OIA. Overall, only 5 specimens were false positive by the ZstatFlu test. Therefore, this test demonstrated high specificity and positive predictive value. In conclusion, the ZstatFlu test is useful for the rapid detection of influenza A and B viruses to identify patients who need antiviral treatment.

Influenza type B neuraminidase can replace the function of type A neuraminidase

Influenza A and B viruses do not form reassortants with each other, presumably due to selection at either the RNA or protein level. Although differences in the promoter sequences of type A and B viruses have been studied, selection at the protein level has not been addressed. In this paper we describe experiments to determine whether differences in structure and/or function of the neuraminidase (NA) protein preclude formation of A/B NA reassortants. Influenza type A (N9) NA or B/Lee/40 NA expressed from plasmids can support multicycle growth of a NA-deficient type A virus (NWS-Mvi), indicating that their function in tissue culture is similar. To determine whether the type A or B NA supplied in trans can be incorporated into the virion of NWS-Mvi, the virus grown in NA-expressing cells was purified by sucrose gradient centrifugation. In each case there was a peak of NA activity coincident with the virus peak, indicating that some NA protein is packaged into the virion. The experiments suggest that, in spite of large sequence differences, the functions of the head, stalk, signal-anchor, and cytoplasmic domains of type A and B NAs are similar in tissue culture. Thus, lack of formation of A/B NA reassortant viruses is not due to restriction at the protein level.